1. Field of the Invention
The present invention relates to a semiconductor device comprising a semiconductor layer containing carbon as a constituent element.
2. Description of the Related Art
A IV group semiconductor containing carbon as a constituent element, particularly SiC, has such a wide band gap as 2.2 to 3.2 eV and, thus, is expected to provide a useful material in the development of a visible light LED or a high speed switching device operating under a high voltage. Particularly, LED of a p-n junction type using the IV group semiconductor permits emitting a blue light and, thus, vigorous researches are being made in an attempt to develop LEDs of a practical level in the field of the visible light LED.
Since the band structure of SiC is of an indirect transition type, the light emission from a LED including a SiC layer must accompany absorption and emission of phonons, with the result that the transition probability between bands for the light emission is very low. Thus, transition between donor (D) and acceptor (A) is utilized in the conventional LED using SiC. A device utilizing the transition between D and A is featured in that the wavelength of the emitted light differs depending on the distance between the lattice positions occupied by D and A. To be more specific, the conventional LED using SiC is defective in that the color tone of the emitted light is rendered poor because the light emitted from the LED has a wide wavelength region. What should also be noted is that, in the case of the D.multidot.A pair light emission, the peak wavelength of the emitted light differs depending on the magnitude of current flowing through the LED. In other words, the color of the emitted light is changed by the magnitude of current flowing through the LED, leading to a serious problem that the display characteristics of a full color LED display element are deteriorated.
It should also be noted that SiC has a wide band gap, as pointed out above. Thus, the energy level of the impurity serving to determine the conductivity type is deep. For example, the energy level of nitrogen (N) SiC has such a deep energy level as about 100 meV. Such being the situation, where SiC is doped with N as a donor impurity, the carrier activation rate relative to the impurity concentration is low. For example, only about 10% of the impurities are activated. It follows that, in order to achieve an n-type crystal growth of a high carrier concentration, it is necessary to set the concentration of the doped impurity at a level scores of times as high as the carrier concentration.
On the other hand, an acceptor impurity Al has such a large activation energy as 200 meV within SiC and, thus, the activation rate is as low as only about 1%. It follows that, in order to achieve a p-type crystal growth of a high carrier concentration, it is necessary to set the concentration of the doped impurity at a level hundreds of times as high as the carrier concentration.
Under the circumstances, extra defects are generated in the case of obtaining an n- or p-type layer of a high carrier concentration. What should be noted is that these extra defects cause marked deterioration in the electrical and optical properties of SiC.
As a doping method which permits overcoming the defects described above, proposed is a technique of forming a shallow impurity level by using a complex center so as to obtain a p-type layer. In this technique, however, it is very difficult to select a suitable dopant.
An additional technique is reported in, for example, "Journal of the Electrochemical Society, III, page 805 (1964)". It is reported that a p-type layer and an n-type layer are obtained by the crystal growth from a solution containing Cr as a main component. In this technique, however, a large amount of Cr exceeding a level of a dopant is considered to remain in the semiconductor crystal, leading to a low crystallity. As a result, the resistivity of the semiconductor layer is not lowered, making the technique impractical, though it is certainly possible to increase the carrier concentration in the semiconductor layer.
As described above, it is difficult to obtain a satisfactory n- or p-type semiconductor layer containing carbon as a constituent element in the conventional semiconductor element, giving rise to the problems that the device is deteriorated by heat generation and that the external quantum efficiency is lowered. Particularly, when it comes to the conventional SiC light emitting device, the color tone of the emitted light is deteriorated by the expansion of the width of the light emission peak. Also, a color change is brought about because the peak wavelength of the emitted light is changed by the magnitude of current flowing through the light emitting device. What should also be noted is that it is difficult to form a p- or n-type layer of a low resistivity in the conventional SiC light emitting device, with the result that the light emitting region of the LED using SiC is restricted to a region close to the electrode. In this case, the emitted light is intercepted by the electrode, leading to a low extraction efficiency of the emitted light.